The oligomerization of light olefins, such as propylene and butenes, to produce higher carbon number olefins, or olefins having 5 or more carbons is known. The oligomerization process is used for the production of high quality motor fuel from low molecular weight olefins. Oligomerization is also referred to as a catalytic condensation process with a resulting motor fuel often referred to as polymer gasoline, or polygasoline. Methods have been sought to improve the quality of gasoline, and in particular the octane number of the gasoline. This octane enhancement is realized through the improvement in reaction selectivity to enhance the amount of high octane blending components as a result of increasing the amount of branched olefins. Polymer gasoline has the benefit of also being a low aromatic content gasoline.
The current state of the conversion of light hydrocarbons to high octane motor fuels involves the use of strong acid catalysts, such as hydrofluoric acid (HF) catalyst, for the alkylation of light paraffins with olefins. This is commonly referred to as HF alkylation. While HF alkylation has a long history in the production of high octane motor fuels, HF alkylation has significant handling issues, and safety concerns due to the nature of hydrofluoric acid. One alternative is sulfuric acid, but this also present issues, and is also a homogeneous catalytic reaction that requires special handling.
The oligomerization process is often combined with other hydrocarbon transformation processes. Other processes include saturation and dehydrogenation. Patents disclosing the dehydrogenation of light paraffins with oligomerization of the olefinic effluent stream include U.S. Pat. No. 4,393,259, U.S. Pat. No. 5,049,360, U.S. 4,749,820, U.S. Pat. No. 4,304,948, and U.S. Pat. No. 2,526,966.
Hydrotreating of olefinic streams to saturate the olefins to produce a high octane fuel is also known. The oligomerization and hydrogenation of a C4 fraction to produce a jet fuel is disclosed in GB 2,186,287, and which also discloses the optional hydrogenation into a premium gasoline. U.S. Pat. No. 4,678,645 discloses the hydrotreatment of jet fuels, diesel fuels and lubricants that have been produced by dehydrogenation and oligomerization of light paraffins. However, hydrotreating of gasoline produced by oligomerization can reduce octane numbers of the gasoline, while saturating olefins to paraffins.
Other known catalysts for effecting oligomerization include heterogeneous catalysts such as boron trifluoride as described in U.S. Pat. No. 3,981,941, or catalysts that are mild protonic acids, generally having a Hammett acidity function of less than −5.0. Particularly preferred are solid phosphoric acid (SPA) catalysts having as a principal ingredient an acid of phosphorous such as ortho, pyro, or tetraphosphoric acid. SPA catalysts can be found in U.S. Pat. No. 5,895,830.
The use of zeolites for oligomerization, and particularly the use of zeolites having medium pores, is also described in the patent literature. U.S. Pat. No. 4,547,613 uses a ZSM-5 type catalyst that has been conditioned at low pressure and high temperature with a light hydrocarbon gas. A process for producing lubricating oils from the conversion of light olefins using the ZSM-5 catalyst is disclosed in U.S. Pat. No. 4,520,221. Other intermediate pore zeolites are disclosed in U.S. Pat. No. 4,642,404 and U.S. Pat. No. 5,284,989.
While work has indicated that zeolites can be used for the oligomerization of olefins, prior use of zeolites produce a poor quality product for use as a gasoline.